The present invention relates to highly crystalline polypropylene resins which are suitable for forming film excellent in transparency, stiffness, impact strength and resistance to blocking.
Hitherto, polypropylene resins have been widely used in the packaging field of food packaging, fiber packaging and the like because of their excellent optical properties, mechanical properties and packaging applicabilities.
As the polypropylene film to be used in the packaging field, a polypropylene having an MFR, Melt Flow Rate, in the range of 1 to 10 g/10 min has hitherto been used in consideration of its moldability. The polypropylene polymerized with a titanium trichloride catalyst was, however, disadvantageous in insufficient transparency of film because of its large or broad Q value such as 5 to 6.
Furthermore, the polypropylene of MFR of 1 to 10 has generally a density as an index of stiffness (what is called nerve) smaller than 0.907 g/cm.sup.3, and has a tendency to have wider molecular weight distribution if polymerization is carried out with a stereoregularity improver for increasing the density. It was therefore very hard to realize both the decrease of the Q value and the increase of the density at the same time.
On the other hand, when a highly active catalyst supported on a magnesium chloride carrier is used, a polypropylene having a narrow molecular weight distribution is obtained. It has, however, a Q value in the range of 4 to 5 which is still insufficient for the improvement of its transparency. Also, it has a density still remaining in the range of less than 0.907 g/cm.sup.3 which cannot be said satisfactory also from the viewpoint of stiffness.
Furthermore, low molecular weight and/or low crystalline polymers in polypropylene (which is a cold xylene soluble matter CXS) which deteriorate the blocking tendency of film were present in large amounts in polypropylenes produced by either of the catalysts used, so that a polypropylene having, in addition to the above-described properties, a small CXS was desired.
Moreover, polypropylene resins which have hitherto been well-known were unsatisfactory in some applications in the balance of stiffness, impact strength and production cost. For instance, for the purpose of improving the stiffness of polypropylene, methods for broadening the molecular weight distribution (Japanese Patent Laid-Open Publication Nos. 2307/1981, 172507/1984 and 195007/1987) and methods comprising adding a nucleating agent to polypropylene (Japanese Patent Publication No. 1809/1964 and Japanese Patent Laid Open Publication No. 139731/1985) have been proposed, but these methods are not desirable because they work for unduly decreasing impact strength notwithstanding the improvement of stiffness.
For the purpose of improving the impact strength, methods comprising copolymerizing with propylene a comonomer such as ethylene or the like (Japanese Patent Publication No. 11230/1968 and Japanese Patent Laid Open Publication No. 35788/1978) and methods comprising blending with polypropylene an ethylene polypropylene random copolymer or an ethylene-butene random copolymer (Japanese Patent Laid-Open Publication No. 43242/1990) have also been proposed. These techniques, however, were disadvantageous in their restricted applications because not only the stiffness of the propylene resin is lowered but also the production cost of film is raised.
As the method for improving the impact strength of polypropylene, there is also known a method for narrowing the molecular weight distribution comprising molecular weight degradation in the course of granulation of a polymer powder obtained by polymerization. However, in the film field, polypropylene is generally required to still have an MFR in the range of 1 to 10 g/10 min even after the molecular weight degradation, and the narrowing of the molecular weight distribution by molecular weight degradation was restricted.